1. Field of the Invention
The present invention relates in general to electric motors and motor particularly to the motors of an axial gap type, which comprises a rotor shaft that is rotatable about its axis, at least one rotor that is fixed to the rotor shaft to rotate therewith and at least one stator that is disposed about the rotor shaft and axially spaced from the rotor.
2. Description of the Related Art
Hitherto, various axial cap motors have been proposed and put into practical use particularly in the field of power generators that need high power density and low thermal generation. One of them is shown in Japanese Laid-open Patent Application (Tokkaihei) 11-187635.
In order to clarify the task of the present invention, one conventional axial gap motor will be briefly described with the aid of FIGS. 10 and 11 of the accompanying drawings.
As is seen from FIG. 10, the axial gap motor 51 comprises a case 55, a rotor shaft 52 rotatably disposed in case 55, an annular rotor 53 fixed to rotor shaft 52 to rotate therewith and a stator 54 arranged about rotor shaft 52 at a position to face rotor 53. Axially spaced two bearings 56 are employed for rotatably supporting rotor shaft 52 relative to case 55. Rotor 53 comprises a rotor back core 57, twelve flat plate magnets 58 and a rotor core 59 which are assembled to constitute one unit.
As is understood from FIG. 11, the twelve flat plate magnets 58 are of a flat plate type and are flatly held by rotor core 58 in such a manner that major flat surfaces of flat plate magnets 58 constitute an imaginary plane that is perpendicular to the axis X of rotor shaft 52. As is seen from FIG. 10, stator 54 comprises a stator back core 60, a stator core 61 and stator coils 62 which are assembled to constitute one unit. As shown, between stator 54 and rotor 53, there is defined a certain gap 63. Near one axial end of rotor shaft 52, there is arranged an encoder 64 that detects a rotation speed (or angular position) of rotor shaft 52. Case 55 is formed with a water jacket 65 through which cooling water flows to cool the motor 51. Rotor back core 57 functions to turn a looped magnetic flux about the axis X of rotor shaft 52. That is, for operating the motor 51, a looped magnetic flux that has passed through one group of flat plate magnets 58 is needed to turn in a circumferential direction for passing through the other group of flat plate magnets 58 and stator 54 next.
FIG. 11 is an enlarged view of a part of rotor 53 taken from gap 63 between stator 54 and rotor 53 (see FIG. 10). As shown, with the presence of twelve flat plate magnets 58, rotor 53 has twelve poles, six N-poles and six S-poles alternately arranged. These flat plate magnets 58 are exposed at their main surfaces to gap 63 that is defined between stator 54 and rotor 53.